Extrusion Shear Induced Degradation Ratios in Recycled Polyethylene Manifold Systems
Maintain manifold shear rates below 300 s⁻¹ and specify 25-minute minimum OIT to prevent catastrophic molecular weight loss in recycled polyethylene extrusion.
Macroradicals
Meaning
High molecular weight polymer chains containing at least one unpaired electron represent the chemical state of macroradicals. These reactive species emerge when covalent bonds within a backbone break during shear or thermal stress. The stability of such structures depends on the surrounding matrix and the availability of kinetic energy to drive further chain reactions.
Polymer Scission
Chain degradation accelerates when these reactive sites propagate through an existing plastic lattice. Mechanical shear during twin screw extrusion frequently triggers this phenomenon by physically pulling molecules until bonds snap. Once the backbone ruptures, the resulting pair of radicals initiates secondary reactions such as crosslinking or further scission.
Unchecked, this mechanism alters the melt flow index of a batch away from target specifications.
Regrind Economics
Excessive use of recycled pellets introduces a high density of pre-existing radical sites into the raw material feed. Virgin resin contains stabilizers designed to quench these reactive entities before they compromise structural integrity. When a processor adds high percentages of regrind, the concentration of broken chains overwhelms the stabilizer package.
Parts produced from this compromised material display reduced impact resistance and premature mechanical failure under load.
Process Stability
Thermal history inside the barrel defines the rate at which macroradicals reach equilibrium in a production run. Stable molding temperatures limit the frequency of random bond cleavage that occurs when heat energy exceeds the covalent bond strength. Maintaining uniform residence time prevents localized overheating where these reactive fragments proliferate uncontrollably.
Proper heat management ensures that the molecular architecture remains within the tolerances required for consistent part performance.